Generally, radiographic images such as X-ray images have been commonly utilized for diagnoses of condition of a patient at medical scenes. In particular, radiographic images by an intensifying screen-film system, as a result of achievement of a high sensitivity and a high image quality during the long improvement history, are still utilized at medial scenes all over the world as an image pick-up system provided with the both of high reliability and superior cost performance. However, the image information is so-called analogue image information, and it is not possible to perform free image processing and electronic transmission in a moment as with digital image information which has been ever developing in recent years.
Therefore, in recent years, a radiographic image detector system such as computed radiography (CR) and flat-panel type radiation detector (FPD) has come to be in practical use. Since these can directly obtain a digital radiographic image and directly display the image on an image display device such as a cathode ray tube and a liquid crystal panel, there is not necessarily required image formation on photographic film. As a result, these digital X-ray image detector systems have decreased necessity of image formation by silver salt photography and significantly improved convenience of diagnostic works at hospitals and clinics.
Computed radiography (CR) has come to be in practical use in medical scenes at present as one of digital technologies of X-ray images. However, the sharpness is not sufficient nor the spatial resolution is, and CR has not achieved an image quality of a screen-film system. In addition, flat plate X-ray detector system (FPD) employing thin film transistor (TFT), described in such as “Amorphous Semiconductor Usher in Digital X-ray Imaging” by John Rowlands, Physics Today, 1997 November, p. 24, and “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” by L. E. Antonuk, SPIE, 1997, vol. 32, p. 2, as a further new digital X-ray image technology has been developed.
Utilized is a scintillator plate, which is prepared by employing an X-ray phosphor provided with a property of emitting via radiation to convert radiation into visible light, and it is necessary to utilize a scintillator plate having a high emission efficiency to improve an SN ratio in image pick-up at a low dose. Generally, the emission efficiency of a scintillator plate is determined by a thickness of a phosphor layer and an X-ray absorption coefficient of a phosphor, however, the thicker the phosphor layer thickness is, the more sharpness is decreased due to scattering of emission light in a phosphor layer. Thus, sharpness desired for an image, which is determined, depends on the thickness.
Since cesium iodide (CsI) particularly had a relatively high conversion ratio from X-ray to visible light and the phosphor can be formed into a columnar crystal structure via vacuum evaporation, scattering of luminescent light in the crystal via an optical guide effect was suppressed, whereby the phosphor layer thickness was possible to be made thicker. However, since an emission efficiency is low with CsI only, as described in Japanese Patent Examined Publication No. 54-35060, an admixture of CsI and sodium iodine (NaI) at a given molar ratio is deposited on a substrate via evaporation as sodium-activated cesium iodide (CsI:Na), or an admixture of CsI and thallium iodine (TlI) at a given molar ratio is recently deposited on a substrate via evaporation as thallium-activated cesium iodide (CsI:Tl), and an annealing treatment is subsequently conducted as a post-process to improve visible region conversion efficiency, and to be used as an X-ray phosphor.
However, there is a problem such that an aging characteristic is deteriorated since a CsI-based scintillator (phosphor layer) exhibits a deliquescent property. In order to prevent such the aging deterioration, it has been proposed that a moisture-resistant protective film is formed on the CsI-based scintillator (phosphor layer) surface. For example, it is known that the upper portion and the side of a scintillator layer (corresponding to a phosphor layer of the present invention), and the outer circumferential portion of the scintillator layer provided on a substrate are covered with polyparaxylene (refer to Patent Document 1, for example). But, since the polyparaxylene resin described in Patent Document 1 exhibits a weak moisture resistance property, the phosphor layer can not be sufficiently protected, and the polyparaxylene resin is penetrated into the spacing of columnar crystals constituting the scintillator layer, whereby there is produced a drawback such that an optical guide effect is blocked.
As to a transparent resin film having a moisture permeability of less than 1.2 g/m2·day, it is known that at least a scintillator layer surface which is not facing a support, and the scintillator layer side are covered (refer to Patent Document 2, for example).
Though the method described in Patent Document 2 is possible to acquire high moisture resistance when a transparent organic polymer film such as polypropylene or polyethylene terephthalate is provided as a protective film via adhesion onto a phosphor layer, it can not be substantially employed as the protective film since there is produced fatal drawback such that sharpness drops, a film thickness of at most 5 μm is required in order to avoid the above-described, and it becomes insufficient to protect the phosphor layer from chemical alteration and physical impact. Though there are methods described in Japanese Patent O.P.I. Publication Nos. 5-312961 and 6-331749 in the case of providing a scintillator panel on the light-receiving element plane, these exhibit low productivity, and degradation of sharpness on the scintillator panel and the light-receiving element plane can not be avoided. An example to use a flexible protective film made of polyparaxylene is described in Japanese Patent O.P.I. Publication No. 2002-116258, but sharpness is degraded because of the after-mentioned reason concerning propagation of luminescent light from the scintillator within the protective film, since the protective film is closely attached to the light-receiving plane when a flexible organic film is used as a protective film.
Further, it is commonly known that a phosphor layer is formed on a rigid substrate made of aluminum or amorphous carbon via vacuum evaporation as a method of manufacturing a scintillator, and the entire surface of the scintillator is covered thereon with a protective film (Japanese Patent No. 3566926).
However, when a phosphor layer is formed on such the substrate which is not freely bendable, there appears a problem such that an even image quality property can not be obtained within the light-receiving element plane of a flat panel detector because of influence from deformation and warpage of a substrate during evaporation, in the case of attaching a scintillator panel to a light-receiving plane. This problem has become more serious while the flat panel detector is growing in size.
In order to avoid this problem, a method of forming a scintillator directly on an imaging element via evaporation is commonly utilized, and a medical intensifying screen exhibiting not much sharpness but flexibility is also utilized as an alternative to the scintillator.
In this situation, it is desired to develop a radiation flat panel detector in which productivity is excellent, degradation of a phosphor layer property during aging is inhibited, the phosphor layer is protected from chemical alteration and physical impact, and sharpness between a scintillator panel and a light-receiving element plane is hardly degraded.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2000-284053.
(Patent Document 2) Japanese Patent O.P.I. Publication No. 2005-308582.